Stephen Stearns opens this segment of his Yale course Evolution and Medicine by asking a question doctors rarely stop to answer: what actually counts as a disease? Drawing on the textbook Evolutionary Medicine, he distinguishes between conditions caused by pathogens, genetic defects, environmental mismatch, and the body's own trade-offs, arguing that a strictly evolutionary definition looks different from a purely clinical one. He works through examples of how natural selection can leave organisms vulnerable to certain illnesses as a byproduct of adaptations that were otherwise beneficial, setting up the course's broader argument that medicine benefits from thinking in terms of function and fitness rather than just mechanism. The lecture runs short, functioning as a framing introduction rather than a deep case study, but it lays out the conceptual vocabulary the rest of the series will build on.